Tetrachloroethene (PCE) and trichloroethene (TCE) are common groundwater contaminants. Under anaerobic conditions these solvents are are transformed by biological reductive dechlorination to dichloroethene isomers (DCEs), vinyl chloride (VC) and sometimes ethene. The original contaminants and their intermediate metabolites are all CERCLA listed hazardous chemicals that are neurotoxic or carcinogenic. Determining the extent of dechlorination under different conditions would be helpful for evaluating the human health risk at individual sites. Ideally, the conditions which promote complete dechlorination to ethene should be ascertained. These conditions are currently unclear and it is this gap in scientific knowledge which this project is designed to address. This project will utilize an enrichment culture developed by Jim Gossett and Steve Zinder at Cornell University, which is known to completely dechlorinate high concentrations of PCE (0.55 mM) and other chlorinated ethenes to ethene. A specific aim of this project is to develop molecular probes that target 16S rRNA genes and reductive dehalogenase genes present in the culture. These probes will be useful for study of the dynamics of the bacterial community and the expression of the dehalogenases in the culture, as electron donors/acceptors and other nutrients are varied. The goal is to find the optimum conditions for dechlorination of PCE, TCE, DCE and especially VC, which accumulates due to its slow rate of dechlorination. The members of the community will be phylogenetically characterized by 16S rRNA typing. Syntrophic relationships between the dechlorinator, Dehalococcoides ethenogenes, and other species will be established by experimentally combining defined members of the community. Presumably, the syntrophic organisms supply electron donor (hydrogen) and nutrients to D. ethenogenes, so these studies will help further define the conditions which produce optimal dechlorination. The presence of other dechlorinators in the Cornell enrichment culture and the anaerobic enrichment culture from Corvalis, Oregon will be assessed by restriction fragment analysis of amplified 16S rRNA, sequencing of selected clones and searching for homology with known dechlorinators. This may lead to the discovery of organisms which dechlorinate VC at a faster rate, or dechlorinate other substrates under different conditions than are optimal for D. ethenogenes.